1. Technical Field
Petroleum bearing formations usually are of high resistivity (low conductivity) due to non-conductive oil or gas having replaced conductive salt-containing water (brine) in the pores of the rock. The resistivity normally is reduced as brine replaces oil or gas during production, or increases if brine is substituted or flushed by fresh water or gas. This difference in resistivity may be used for monitoring a reservoir during production, or for distinguishing between petroleum-bearing and brine-bearing parts of a formation.
2. Description of Related Art
Generating an electromagnetic signal from a towed antenna is used for exploring sedimentary seabed formations below the seafloor. A typical resistivity of seawater, ρsw, may be about 0.3 Ωm, which is rather conductive. Attenuation of electromagnetic wave propagation is thus very high. This high conductivity (or low resistivity) of the seawater prevents much of the EM signal that is transmitted from the towed antenna in the water masses to reach even the seafloor. A rather small, attenuated portion of the energy that reaches the seafloor propagates further through the rock beds, some of it downwards. The wave energy is partly reflected, partly refracted by formations having varying electromagnetic properties, and some of the energy propagates back to the surface of the seafloor. At the seafloor the EM waves must be detected by means of EM receivers, and then analyzed.
A Norwegian Patent, NO 310 383 to Statoil, describes electromagnetic guided wave propagation along a conductive string in a well. The guided wave is reflected by resistivity transitions in a petroleum production well. Due to oil being replaced by a brine water front from below, near the production well, repeated measurements of such reflected guided waves are used for monitoring the rising water front during a considerably long time span, e.g. during months or years. No wave propagation far from the well is considered; only the immediate near-well volumes of fluids are measured.
Norwegian Patent application NO 20020203 from Statoil describes a method and an apparatus for determining properties of an underground reservoir. That application mentions a transmitter antenna arranged in a well penetrating a formation, in which well the transmitter antenna is arranged near the formation of interest. NO 20020203 mentions that the casings pipe may be used as an antenna, in that a portion of the casing is insulated above the portion actually used for constituting a dipole transmitter antenna. The casing must be insulated and modified for the method to work. A power supply is lowered into the well casing and forced towards the inner surface of that casing. A significant disadvantage of NO20020203 in case of arranging the antenna in the well is due to the fact that the operation to lower the antenna to the required depth in the well is relatively complicated. Additionally, the operation of lowering the antenna inside the well most often requires that in such a well, in case of being a production well, production must be shut down temporarily. Providing electrical energy for such a downhole transmitter antenna arranged near the actual production zone may be difficult, requiring long electrical supply lines. For a well being drilled, the operation of arranging a downhole transmitter antenna would most probably be out of the question due to drilling economy and power supply problems. Another significant problem is mentioned in Statoil's Patent application in page 4, line 7 to 8: Pulses are subject to strong dispersion in the conductive medium, i.e. while propagating from the surface, downwards through the overburden conductive sediments. This means that not much signal remains to return to the surface to be measured.
Patent application NO 20020203 discusses insulating the transmitter antenna, i.e. a part of a conductive casing. Below, we will describe that it is feasible to use a non-insulated transmitter antenna on the seabed, and still achieve a significantly better signal transmission.
Another Patent application, NO20020202 from Statoil, also discusses delineating a hydrocarbon bearing rock layer, presumably of high-resistivity, using refracted EM waves from guided waves in that rock layer. NO20020202 correctly recognizes that the detected refracted waves may be less attenuated than a direct EM wave or large offsets, but does not discuss using a transmitter antenna at the upper part of a casing with the casing acting as signal guide down to the reservoir.
NO20020202 states that a towed dipole antenna having a length from 100 to 1000 meters is preferred.
International Patent application WO 0157555 from Den norske stats oljeselskap as and Norges Geotekniske Institutt: “Method and apparatus for determining the nature for subterranean reservoirs”, describes a transmitter antenna arranged at the seafloor, and a corresponding receiver antenna also arranged at the seafloor. This set-up is made for investigating a deep reservoir layer otherwise known from seismic surveys. A refractive wave component is sought in the wave field response, to determine the nature of the reservoir. WO 0157555 stresses the fact that a propagating electro-magnetic wave through a hydrocarbon [rock] layer is much less attenuated than an EM wave propagating in a water-bearing stratum, while the speed is much higher in the hydrocarbon-bearing layer. Thus, far from the transmitter antenna, the refracted wave will be far less attenuated than the reflected wave, or a direct wave. WO 0157555 also mentions at page 4, line 33 that the transmitter may be inside an existing well. WO 0157555 also mentions that the reservoirs of interest may be about 1 km or more below the seabed, and continues saying “In order to carry out electromagnetic surveying as a stand alone technique in these conditions, with any reasonable degree of resolution, short wavelengths are necessary. Unfortunately, such short wavelengths suffer from very high attenuation. Long wavelengths do not provide adequate resolution. For these reasons, seismic techniques are preferred”.